Method of preventing motion of article in an article holder

ABSTRACT

The present invention comprises a dynamic brake that applies restraining frictional force to a wafer in a wafer holder while the wafer holder is substantially at rest, but releases the restraining force as the processing carousel containing several wafer holders rotates about a central axis of the carousel. This dynamic brake preferably comprises a boot that passes through an opening in the wafer holder to rest on the surface of the wafer in an exclusion zone near the wafer&#39;s edge. The exclusion zone is typically no more than about 3 mm in extent. The frictional force between the boot and wafer is sufficient to prevent unwanted motion of the wafer in the holder. As the wafer holder rotates about a central axis of the processing carousel, centrifugal forces applied to the brake arising from such rotation cause the boot to pivot upward, releasing the frictional force on the wafer.

CROSS REFERENCE TO RELATED APPLICATION

This application is a division of and claims priority from U.S. patentapplication Ser. No. 09/456,094, filed Dec. 7, 1999, entitled, “DynamicBreak For Non-Contact Wafer Holder”.

BACKGROUND OF THE INVENTION

1. Technical Field

The present invention relates to the processing of flat panels,integrated circuits or other wafer-like objects in a non-contact holderand, more particularly, to apparatus and methods for preventing unwantedmotions of the object within the holder.

2. Description of Related Art

Wafer-like objects (hereinafter referred to as “wafers”) often require ameans to hold the objects in position while performing processing orother operations thereon. Non-contact wafer holders may make use of acushion of gas (typically air) to maintain a separation between theholder and the object being processed. Therefore, motion of the objectin the plane of the holder is virtually frictionless, unless forces inaddition to the air cushion are supplied. Holding the object in positionin a non-contact holder while avoiding unwanted and possible dangerousmotions in the plane of the holder, are among the primary goals of thepresent invention.

To be definite, we will describe the common instance of the processingof semiconductor wafers as may typically occur in the production ofintegrated circuits or in wafer post-processing for thinning, etc.However, the processing of any wafer-like object subject to unwantedmotion while in a non-contact holder is also a potential area ofapplication for the present invention. Flat panel displays and otherrectangular, circular, star-shaped or irregularly shaped planar objectsmay require mounting in a holder for processing without the risk ofdamage to the object by contact with the holder. For economy of languagewe will refer to all such wafer-like objects herein as “wafers”recognizing that such objects may be large (perhaps having a size up tothe order of one meter or larger) and need not be rectangular, circularor regular in shape. Since semiconductor wafers are expected to be animportant area of application for the present invention, we willdescribe the primary features of the present invention in terms ofsemiconductor processing, not intending to limit the invention to thisparticular choice or particular example. Semiconductor wafers aptlyillustrate the features of the present invention and permit obviousmodification for use in processing other wafer-like objects.

Integrated circuits are typically fabricated on a wafer of silicon withnumerous integrated circuits fabricated on a single wafer. This processrequires the wafer to be held in position for the multiple processingsteps required to complete circuit fabrication and for processing stepsfollowing fabrication of the integrated circuits. However, intimatemechanical contact between the wafer and its holder incurs the risk ofdamage to the wafer or the devices fabricated on the wafer's surface.This risk of damage increases as wafers become thinner and easilydistorted during processing. Thus, wafer holders that do not requiremechanical contact between wafer and holder have been developed.

The floating of a wafer above a layer of compressed gas is described inthe work of Pirker (U.S. Pat. No. 5,896,877). The wafer is held inposition by gravity while the air cushion prevents contact with thewafer holder. Work of Siniaguine and Steinberg (PCT InternationalPublication No. WO 97/45862) describes a non-contact holder forwafer-like objects in which a vortex of rotating air provides both thevacuum support for the wafer and the air cushion.

When the wafer holder is at rest during loading or unloading, the waferis not stable in the holder. Non-contact wafer holders necessarilyprovide forces positioning the wafer in a direction perpendicular to theholder. Gravity is used in the work of Pirker while a vortex-createdvacuum is used by Siniaguine et. al. However, both approaches tonon-contact wafer positioning provide minimal hindrance to the wafer'smotion parallel to the surface of the holder. Random forces derivingfrom the use of an air cushion, or from small rotations of the holderduring start-up and shut-down, may result in torques being applied tothe wafer. Such torques may lead to undesirable motion of the wafer inits holder and the possibility of wafer damage.

Integrated circuits are typically fabricated in a multi-wafer holder or“carousel,” rotating through a plasma for thinning. When the carouselcontaining many wafers begins to rotate, centrifugal forces press eachwafer against limiting pins. A moving wafer contacting limiting pins maychip or perhaps fracture. Damage is more likely if the wafer is thin,typically about 50 microns. Procedures and devices for preventing suchunwanted wafer motions during loading and unloading of the carousel arean example of the applicability of the present invention.

BRIEF SUMMARY OF THE INVENTION

Non-contact wafer holders may use vacuum or gravity to keep the wafer inclose proximity to the wafer holder while maintaining a separation bymeans of a gas or air cushion. However, undesired motion of the wafer inthe plane of the wafer holder may build up to dangerous rates if notprevented. As wafer holders in a processing carousel rotate about acentral axis of the carousel, centrifugal forces are commonly used tohold the wafer firmly against a limiting device, preventing unwantedseparate motion of the wafer in its holder. However, after loading ofthe wafer into the holder but before centrifugal forces arise due tocarousel rotation, the wafer may still build up dangerous levels ofin-holder motion. Similar risks are encountered during wafer unloading.The present invention comprises a dynamic brake that applies restrainingfrictional force to a wafer in a wafer holder while the wafer holder issubstantially at rest, but releases the restraining force as theprocessing carousel containing several wafer holders rotates about acentral axis of the carousel. This dynamic brake preferably comprises aboot that passes through an opening in the wafer holder to rest on thesurface of the wafer in an exclusion zone near the wafer's edge. Theexclusion zone is typically no more than about 3mm in extent. Thefrictional force between the boot and wafer is sufficient to preventunwanted motions of the wafer in the holder. As the wafer holder rotatesabout the central axis of the processing carousel, centrifugal forcesapplied to the brake arising from such rotation cause the boot to pivotupward, releasing the frictional force on the wafer as the samecentrifugal forces cause the wafer to press firmly against the limitingpins.

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings herein are not to scale.

FIG. 1A: A schematic bottom view of a wafer processing carousel.

FIG. 1B: A schematic top view of a wafer processing carousel.

FIG. 2: A schematic cross sectional view of the brake of the presentinvention in its lowered position.

FIG. 3: A schematic cross sectional view of the brake of the presentinvention in its raised position.

DETAILED DESCRIPTION OF THE INVENTION

In the following description and figures, similar reference numbers areused to identify similar elements.

FIG. 1A depicts a schematic view (not to scale) of a typical waferprocessing apparatus commonly referred to as a “processing carousel,” 1.The view depicted in FIG. 1A shows the processing carousel, 1, from theside containing the wafers, 3. The vacuum non-contact wafer holder ofSiniaguine et. al. typically supports the wafer by means of suctionapplied from above generated by vortex motion of air or other gas. Theholder of Pirker uses weight of the wafer itself to hold the wafer inproximity to the holder. In both cases, a separation between wafer andholder is maintained by means of an air cushion. The present inventionis applicable to non-contact wafer holders supporting the wafer fromabove in the manner of Siniaguine et. al. Thus, we depict in FIG. 1A theview from directly below the wafer holder.

FIG. 1A depicts two segments, 2, of the processing carousel, 1, withwafer holding locations therein, 3. The entire 360° circumference ofprocessing carousel, 1, will typically be divided into segments, 2, andwafer holders, 3. Only two are depicted in FIG. 1A.

Wafers are typically loaded into holders, 3, with processing carousel,1, being substantially at rest. Non-contact wafer holders interpose anair cushion between the wafer and the holder providing essentially noforce to prevent the wafer from moving in a direction parallel to theplane of the holder. Typically, gas will exit from the air cushion moreon one side than another inducing a torque in the opposite directionpursuant to Newton's laws of motion. Thus, wafers in the holders maycommence rotation, depicted as 4 in FIG. 1A. Furthermore, since thesource of torque resulting in rotation is typically unpredictablenon-uniformities in air flow, the direction of rotation may be differentfor different wafers in the processing carousel as denoted by oppositechoices for 4 in FIG. 1A. With essentially no frictional force hinderingthe rotation of the wafer in its holder, the speed of rotation may buildto dangerous values. Contact of a rapidly rotating wafer with limitingdevices (12 in FIG. 2) may lead to damage of the wafer. Prevention ofundesired rotation in non-contact wafer holders is one objective of thepresent invention.

For economy of language herein we use “wafer rotation” to signifyundesired motions of the wafer with respect to its holder andsubstantially in the plane of the holder, including but not limited tocomplete rotation of the wafer. It is understood that wafers may undergovarious types of rotational and non-rotational motion in the holder, anyof which raise the possibility of wafer damage upon contact with thelimiting pins or inner surfaces of the holder. The prevention of anysuch motion is included within the scope of the present invention.

A common mode of semiconductor wafer processing has processing carousel,1, rotate about its central axis, 5, perhaps through one or moreprocessing zones. Such rotation of the processing carousel, in direction6 for example, leads to a radially-outward centrifugal force on thewafers. This centrifigal force holds the wafers against suitablelimiting pins or similar devices, 12, preventing rotation or otherundesired motion of the wafer in its holder. Thus, the present inventionrelates to a brake preventing rotation of the wafer in its holder whenthe processing carousel is at rest (or nearly so), but releasing whenthe processing carousel executes rotational motion.

FIG. 1B views the processing carousel and wafer holders, 3, from theopposite face from the view in FIG. 1A. That is, FIG. 1A views from theside of the processing carousel containing the wafers, FIG. 1B views theprocessing carousel from the side opposite that containing the wafers.The brake of the present invention is positioned on the top (opposite)face of the wafer holder from the wafer, and overlapping the position ofthe wafer, as depicted schematically by 7 in FIG. 1B.

FIG. 2 depicts in cross sectional view one embodiment of the dynamicbrake of the present invention. Wafer, 9, is held in close proximity toholder, 8 by means of an upward-directed vacuum force, with an aircushion maintaining the separation between wafer and holder. Anexclusion zone of no more than about 3 mm, 11, on the edge of the wafer,8, is used in the practice of the present invention, causing the braketo rest in this zone devoid of circuit components. The “exclusion zone”is traditionally reserved for wafer handling purposes. If futuregenerations of integrated circuits decrease the size of the exclusionzone, the dimensions and location of the brake of the present inventionwill be readily modified to accommodate. A boot, 10, extends through anopening in holder, 8, to make contact with wafer, 9. The contact ofboot, 10, and wafer, 9 provides sufficient friction on the wafer toprevent wafer rotation, while making contact with a very limited regionof the wafer.

FIG. 3 depicts the cross sectional view of FIG. 2 with processingcarousel rotating about its central axis, 5. Rotation of processingcarousel, 1, induces a centrifugal force, 18, directed radially outwardtowards the circumference of the wafer holder. The brake of the presentinvention, 13, has its center of mass lying above the pivot axis, 16.Therefore, centrifugal force, 18, causes the brake to rotate about pivotaxis, 16, in a clockwise direction, 19. Rotation, 19, causes boot, 10,to rise away from the wafer as depicted by 20. A pivot axis for the bootis provided by 15 so boot, 10, may freely rise away from the wafer.While pivot 15 is preferred in the practice of the present invention, itmay be omitted in brake designs providing sufficiently smooth extractionof boot 10 away from the wafer and through the wafer holder without suchpivot.

When thus raised, boot, 10, is no longer providing frictionalstabilization against rotation of the wafer in its holder. However, thesame centrifugal force causing brake, 13, to rotate away from the waferalso causes the wafer, 9, to be firmly held against limiting device 12.Thus, the brake of the present invention rotates out of the way as thecentrifugal force on the wafer itself prevents wafer rotation bycreating contact between the edge of the wafer and a limiting device

After having all wafer holders filled with wafers, processing carousel,1, typically begins to accelerate in angular direction, 6, until theoperational angular velocity is reached. Let R_(min) be the minimumrotational (angular) velocity of processing carousel, 1, necessary tohold wafer, 9, snugly against limiting devices, 12. That is centrifugalforce on the wafer at R_(min) is sufficient to prevent rotation of thewafer within the wafer holder by means of pressing the wafer againstlimiting devices, 12. The brake assembly, 13, should be designed withappropriate weight and balance to rotate boot, 10, away from wafer, 9,at a rotational velocity comparable to R_(min). Precision is notnecessary in adjusting the rotational velocity at which boot, 10,elevates away from wafer, 9. It is merely necessary that boot, 10, holdwafer, 9, in position for sufficient time. “Sufficient time” as usedherein means that the time between release of wafer, 9, from thefrictional force of boot, 10, and the stabilization of wafer 9 againstlimiting devices, 12, is too short for the wafer to build up dangerousangular velocity in the wafer holder. This period of time is determinedby the detailed dynamics of the devices and can readily be determined bytesting. It is often convenient to include as part of brake 13 abalancing weight, 17, to insure that boot, 10, promptly descends andfirmly makes contact with wafer, 9, upon deceleration of the processingcarousel.

As the carousel slows following wafer processing, the brake descendswhen the lifting centrifugal force no longer suffices to keep the bootelevated. Thus, the present dynamic brake invention automaticallyprevents rotation of the wafer in its holder for both loading andunloading.

The present invention is not limited to the processing of semiconductorwafers and, indeed, may process wafer-like objects of relatively largesize. There is no inherent mechanical limitation in the presentinvention for processing flat panel displays or other wafer like objectsup to about 1 meter in lateral dimension or larger. There is norestriction in the practice of the present invention that the wafer becircular, rectangular or have any regular shape at all. However, forirregularly shaped objects it is important to position the dynamic brakeof the present invention at locations (as depicted in FIG. 1B) such thatboot, 10, always lies upon a wafer. That is, when processing irregularobjects, certain orientations of the object may cause boot, 10, to missthe wafer entirely and descend to a position even with or below thewafer, 9. Should this occur, impact of wafer, 9, with such an abnormallydescended boot may damage the wafer. The preferred embodiment of thepresent invention is to locate the dynamic brake at a location 7 suchthat boot, 10, will contact wafer, 9, no matter how wafer, 9, may beoriented in the holder. Alternatively, guides may be provided such thatthere is no variation possible in orienting a wafer in the holder, evenfor irregularly shaped wafers, such that boot, 10, always descends upona wafer in the same relative location. Yet another possible solution isto provide a “stop” mechanism preventing boot, 10 from descending belowthe plane of wafer, 9.

It may be useful in the practice of the present invention to limit thegas flow around boot, 10. Therefore, another embodiment of the presentinvention may include a sealing ring, 21, in the form of an o-ringlimiting gas flow around boot, 10, without significantly hindering themotion of boot, 10, in its channel.

The present invention is not limited to rotational motion of amulti-holder carousel. A wafer processing scheme in which each separatewafer undergoes individual rotation may also make use of the presentinvention. The utility of the present invention lies in that it preventsundesired motion of the wafer when loading or unloading from asubstantially stationary wafer holder. But centrifugal forces brought tobear upon rotation of holder, carousel or other structure are felt bythe brake of the present invention, leading to its release from thewafer.

Having described the invention in detail, those skilled in the art willappreciate that, given the present disclosure, modifications may be madeto the invention without departing from the spirit of the inventiveconcept described herein. Therefore, it is not intended that the scopeof the invention be limited to the specific and preferred embodimentsillustrated and described. Rather, it is intended that the scope of theinvention be determined by the appended claims.

What is claimed is:
 1. A method for preventing motion of an article inan article holder, the method comprising: supporting the article by theholder through use of a vortex motion of a gas, with a gas cushioninterposed between the article and the holder; extending a brake memberfrom the holder towards the article to contact the article when thearticle is being loaded into the holder and/or unloaded from the holder,wherein the brake member is attached to the holder; and withdrawing thebrake member from the article between the loading and unloadingoperations.
 2. The method of claim 1 further comprising, after theloading operation, rotating said article holder causing thereby acentrifugal force to press said article against a limiting devicefixedly attached to the holder; wherein the contact of said brake memberwith said article is terminated substantially contemporaneously withsaid pressing of said article against said limiting device.
 3. Themethod of claim 2 whereby said contact is terminated as a result ofmoving said brake member away from said article by said centrifugalforce.
 4. The method of claim 1, wherein the article is a semiconductorwafer.